Rail guns are devices that launch a projectile by the magnetic (Lorenz) driving force of current carried between spaced, parallel conductive rails. The conductor used to pass the current across the space between the rails is called the armature. Four types of armatures are used. A solid armature is a body, at least partially composed of metal, conformed to the rails to make sliding metal/metal contact with the rails during launch. A plasma armature consists of ionized gases. It is typically triggered by vaporizing, via the launching current pulse, a metal foil spanning the gap between the rails. After having been started, the heat of the plasma discharge generates additional ions from surrounding surfaces. A transitioning armature is designed to function as a solid armature until it melts through bulk joule heating, after which a plasma armature takes over automatically. A hybrid armature will typically start as a solid armature but is designed to complete the launch by the establishment of a layer of plasma discharge between it and the rails on either side in lieu of solid/solid metal sliding.
The relative advantages and disadvantages of these types of armatures are these. Among the four types, solid armatures cause the smallest evolution of Joule heat at the armature/rail interfaces but waste the most friction heat. Below the speed at which gouging will be expected, which is between about 1 km/sec and 2 km/sec depending on choice of materials, they may cause little rail damage. Rail damage is of paramount importance because rail guns should be capable of firing up to thousands of times. It is probable that a speed limit exists beyond which these solid armatures cannot be accelerated, probably again between 1 km/sec and 2 km/sec. If not integrated with the projectile so that the current conducting metal at the same time performs a structural function, the solid armature represents useless mass that must be accelerated, named "parasitic mass".
Plasma armatures waste a maximum of Joule heat among the four types of armatures but are associated with a minimum of friction heat and of parasitic mass. They also have a great potential for damaging the rails.
Transitioning armatures are constructed to disintegrate, through melting on account of bulk joule heating, in some predetermined velocity range. Transitioning armatures are a different class from hybrid as well as solid armatures because the solid armature part thereof is designed to survive only to a speed well below the intended final speed of the payload. They have the advantages and disadvantages of the solid and plasma armature, before and after transitioning, respectively.
Hybrid armatures are designed for a short period of solid metal/metal sliding, and then establish a plasma between the solid armature body and rails. Much of the solid part of the armature survives the launch, however. Compared to solid armatures they waste Joule heat, with typical voltage drops across the two plasmas in the range of 100 volts as compared to only several volts across sliding metal/metal contacts, but suffer less frictional losses. Rail damage is typically slight, and achieved speeds are higher than with solid or transitioning armatures.
In rail gun operation, an armature is accelerated from the breech toward the muzzle end of the barrel. It travels in the bore defined by the rail surfaces and by insulators which are disposed between the rails to prevent current flow from one rail to another without passing through the armature. The armature may push a projectile or other payload ahead of it. Alternatively, the armature itself may serve as a projectile, or it may be integrated with one or more projectiles and/or other payloads. These configurations are called armature/projectiles.
For an armature/projectile pushed from its breech end, the pressure due to the inertial force in the armature/projectile must not exceed its material fracture strength. As a result, the length of armature/projectiles is limited by the desired acceleration. As maximum attainable acceleration and desired velocity dictate barrel length, these factors determine the minimum barrel length required to launch the projectile. To the extent that maximum acceleration is limited by projectile material strength, rail guns with long, unwieldy barrels are needed to accelerate armature/projectiles to the desired high velocities.
Another problem with armature/projectiles is related to the magnitude and distribution of current. The accelerating force on the armature/projectile is directly related to the square of the current. Thus, high currents are required to accelerate the armature/projectile to useful velocities over acceptable barrel lengths. As a result, the rail guns must be externally supplied with extremely high currents.
As rail guns are being developed for applications requiring ease of transportation and aiming, and/or rapid firing, the demonstrated needed long barrel lengths and high currents pose a problem. Proposed uses for rail guns include terrestrial anti-tank guns and launchers and space-based anti-missile guns and launchers. Since Joule heating is proportional to the square of the current, the current-carrying components outside of the gun need to be correspondingly massive. Similarly, electric storage devices and switches also need to be massive.
In order to reduce the current, two modifications from the basic design above can be employed; augmented rail guns and stacked rail-guns. Both use multiple, parallel rails in two rail sets that replace the two single rails considered so far. In the augmented gun these are placed flat-on, side by side, with only one pair of rails along the bore. In the stacked gun, the rails are stacked on top of each other and each one forms part of the bore.
In the case of the side-by-side rails of the augmented gun, the current passes from the breech end up one inside rail, through the armature, and back down to the breech end of the other inside rail. The basic rail gun circuit thus completed, the current is then passed from the breech end of the second inside rail through a lead into the breech end of a rail parallel to and aligned with the first inside rail. From the muzzle end of that, the current is passed through another lead into the muzzle end of a corresponding rail on the opposite side and out its breech end. After forming this one extra current turn about the bore, one or more further such turns can be made by feeding the current through another lead in to the breech end of the next rail on the other side of the bore, the rails being successively displaced from the bore by one additional rail thickness. The magnetic field of the additional current turns increases or augments the magnetic field inside the bore, and thus increases the Lorenz force acting on the armature. The force is increased approximately in proportion with the number of rails used.
A multi-turn stacked rail gun has at least two pairs of rails exposed to the bore, as contrasted to the single exposed pair of the simple rail gun and the augmented gun. These rail pairs are stacked together into what basically is an n-turn solenoid. Each turn consists of the length from breech end to armature of one of a pair of rails, a part of the armature, from here on called a "stage", and the length from armature to breech end of the other rail of the pair, the turn being completed through a connecting lead to the first rail of the next pair at the breech end. The armature carries the current in each turn in separate, mutually electrically insulated stages which are stacked up in a direction normal to the direction of motion and are embedded in a single insulating block that together with the conducting parts completes the rail gun armature. This type of multi-turn gun was illustrated by J. G. Moldenhauer and G. E. Hauze, Proc. 2nd Symp. on Electromagnetic Launch Technology, Boston Oct. 10-18, 1983 (IEEE New York, 1983) p. 85-88.
For the same force and the same dimensions of the bore, the stacked rail gun theoretically needs only 1/nth the current of a single turn gun if there are n turns. However, neither the augmented nor the stacked multi-turn rail gun have improved the state of the art in relation to the maximum achievable projectile velocity at a given barrel length because of the limitation created by the material strength of the armature/projectile.
An additional problem arises due to the current skin effect, which causes the current flow lines carried through an armature to crowd in the tail or breech end of the armature. Because the current flows for only a very short time on the order of 0.0001 seconds, the current is able to penetrate only superficially into the armature. This shallow penetration depth has two consequences. First, local hot spots form and the material can overheat. In fact, the temperature increase in the breech end through high current density can melt most potential armature materials very quickly. This is a severe drawback in non-transitioning solid or hybrid armatures. Second, the unequal current distribution causes the force to be applied very non-uniformly, even in a simple armature which does not push a projectile. At least at the start of a launch the current crowds in the tail of the armature, and the Lorenz force is concentrated there. This condition leads to very high stress and material failure problems.
An improved armature, disclosed in U.S. Pat. No. 4,430,921, is theoretically designed to eliminate the hot spots and high current zones at the tail or breech end. The armature is made from multiple laminations with the conductivity of the laminae increasing from the tail to the nose or muzzle end. The conductivity is proposed to be graded to achieve uniform current distribution in rectangled armatures. This grading of electrical conductivities can be used with planar or chevron-shaped laminations.
Besides avoiding graded bulk Joule heating, such grading of electrical conductivity would ideally apply the electrically induced driving force more uniformly along the length of the armature. However, since the driving force is proportional to the square of the current density while the inertial force builds up linearly from the nose to the tail, if an armature/projectile of uniform material distribution is used, a uniform current distribution will only partially balance the inertial force, even if there is no separate projectile being pushed by the armature from its breech end. For more perfect relief of internal stress, the current flow lines must be moved from the tail and crowded towards the nose of the armature/projectile to generate a relative or absolute maximum of current density away from the tail. The prior art armatures can not accomplish this current distribution, largely for the reason that in fact most of the armature resistance resides at the rail/armature interfaces and not in the bulk of the armature.
Another problem with the design of U.S. Pat. No. 4,430,921 is that the armatures fail through detachment of the rear leaf. This has at least two causes. First, as already indicated, the bulk resistance of solid armatures is typically much smaller than the interfacial resistance between the armature and the rails, so that relative differences of bulk conductivity in the armature can not materially affect the current distribution in them unless resistivities in the bulk are made uneconomically high. Second, the crowding of the current into the rear leaf or leaves stimulates the penetration of magnetic flux into the gaps between the leaves, causing a corresponding pressure between them which leads to detachment of the rear leaf. A similar problem frequently occurs if an armature pushes a non-conducting projectile, in that an arc is prone to penetrate into the crack at the front end of the armature, leaving the armature behind and propelling the projectile only with a plasma.
To provide a rail gun in which the armature can be accelerated to optimal velocities, then, the current flow must be distributed by means other than grading of resistivities such that the electrically induced driving force balances the inertial force. This reduces internal stresses and hot spots, and allows acceleration to be limited only by the material strength of the rail gun itself, not the armature. The result is the ability to accelerate an armature to desired velocities with a rail gun that is small and light enough to be a practical launcher or weapon. In addition, the shorter travel times would also reduce ablation and rail damage, reduce friction losses, and improve energy conversion efficiency.